Recently, a number of new classes of molecules have been studied as short wavelength lasers, the reactions being driven by flash tubes and/or electron beams. These molecules have included many of the noble gas halides such as XeBr; the noble gas and halide molecules themselves, iodine-containing compounds such as CF.sub.3 I and C.sub.3 F.sub.7 I; the column 6 excited elements such as O(.sup.1 S.sub.o) and S(.sup.1 S.sub.o); and a sub class of cyanogens including CH.sub.3 CN. All of these compounds respond as primary lasers, although the associated efficiencies are sometimes low. The pure noble gas, pure halides and noble gas-halide mixtures emit radiation at wavelengths of 1100-3400 A, which is substantially the same wavelength range at which the other molecules, such as C.sub.3 F.sub.7 I, N.sub.2 O and CH.sub.3 CN (which produce I*, O(.sup.1 S.sub.o) and CN* or CN.sup.+, respectively) absorb in undergoing laser action. The subject invention uses the fluorescence radiation of the noble gases and/or halides to photolytically drive these latter three classes of compounds, using primary and secondary pumping to achieve high efficiency laser action with potentially high energy storage capacity.
In U.S. Energy Research and Development Administration technical report No. UCRL-51455, there is considered the possibility of fabricating a high energy storage visible laser using the excited metastable .sup.1 S.sub.o - .sup.1 D.sub.2 transition in oxygen and the other column 6 elements, sulfur, selenium, and tellurium. It was found that oxygen and other column 6 elements are most efficiently excited by photolysis, at partial pressures less than 100 torr, using 9.8 eV photons for O(.sup.1 S.sub.o) with lower energies sufficing for the higher atomic weight elements (e.g., 7.3 eV for Se(.sup.1 S.sub.o). The .sup.1 S.sub.o - .sup.1 D.sub.2 transition in oxygen, for example, yields radiation of wavelength 5577 A, with somewhat longer associated wavelengths for the corresponding transition in the sulfur, selenium, and tellurium. Thus, the column 6 elements are good candidates for the laser gas if the secondary source can provide radiation of about 7-10 eV.
There have been experimental studies of the use of Ar.sub.2 * radiation to generate O(.sup.1 S.sub.o) by photodissociation of NO.sub.2.
Another class of candidates for the laser gas is the cyanogens, especially CH.sub.3 CN, HCN, ClCN, BrCN, ICN, CF.sub.3 CN, C.sub.2 F.sub.5 CN and (CN).sub.2, which have been shown to behave as photodissociation and predissociation chemical lasers by West and Berry in Journal of Chem. Phys., Vol. 61, p. 4700 (Dec. 1974), and Journal of Chem. Phys., Vol. 59, p. 6229 (Dec. 1973). Working at cyanogen partial pressures of 0.5-15 torr, West and Berry produced laser action by flash photolysis of the cyanogens at wavelengths greater than 1550 A. Thus, the abovementioned group of cyanogens will also work as laser gases, if pumped by an appropriate short wavelength source.
Flash photolysis (.lambda.- 2680 A) laser action in CH.sub.3 I, CF.sub.3 I, C.sub.2 F.sub.5 I, C.sub.3 F.sub.7 I, and C.sub.4 F.sub.9 I at 1-100 torr has been reported by Kasper et al in Applied Physics Letters, Vol. 5, p. 231 (December 1964) and in Journal of Chemical Physics, Vol. 43, p. 1827 (September 1965), where laser emission from excited iodine I* was found to extend over times of the order of 20 .mu.sec, long after the flash lamp had peaked and declined. Gensel et al, in Applied Physics Letters, Vol. 18, p. 48 (January 1971), reported that application of an inhomogeneous magentic field along a laser tube containing CF.sub.3 I results in Zeeman broadening of the spectral lines by an amount proportional to field strength B and results in an increase of laser energy storage capacity by perhaps 60% by limiting the onset of superfluorescence. Gensel et al also reported the existence of possible chemical pumping via reactions such as CF.sub. 3 + CF.sub.3 I .fwdarw. C.sub.2 F.sub.6 + I* which augments photolytic response. Zuev et al, in Soviet Physics JETP, Vol. 35, p. 870 (November 1972), and Alridge in Applied Physics Letters, Vol. 22, p. 180 (February 1973) and I.E.E.E. Journal of Quantum Electronics, p. 215 (May 1975), reported that addition of an inert buffer gas such as He or Ar to gases such as CF.sub.3 I and C.sub.3 F.sub.7 I results in homogeneous pressure broadening which lowers the stimulated emission cross section o.sub.e but leaves the inversion population density .DELTA.n unchanged; this reduces the gain per round trip of the lasing medium and thus increases laser energy storage capacity. These results indicate that photolysis of CH.sub.3 I and of C.sub.n F.sub.2n + 1 I results in lasing action, and that laser energy storage density may be quantitatively controlled by increased buffering or by inhomogeneous magnetic fields.
Birich et al in Soviet Physics:JETP Letters, Vol. 19, p. 27 (January 1974) have replaced carbon in the Se.sub.2 iodine compounds by the column 5 elements, phosphorous, arsenic and antimony, and have obtained laser action from the decay of I* in at least 16 compounds: (CF.sub.3).sub.2 AsI, CF.sub.3 (C.sub.2 F.sub.5)AsI, CF.sub.3 (C.sub.3 F.sub.7)AsI, (C.sub.2 F.sub.5).sub.2 AsI, (C.sub.3 F.sub.7).sub.2 AsI, (CF.sub.3).sub.2 PI, CF.sub.3 (C.sub.2 F.sub.5)PI, CF.sub.3 (C.sub.3 F.sub.7)PI, (C.sub.2 F.sub.5).sub.2 PI, (C.sub.3 F.sub.7).sub.2 PI, CF.sub.3 (CH.sub.3)PI, CF.sub.3 PI(CN), CF.sub.3 (CF.sub.2 Cl)PI, F.sub.3 PI, OPF.sub.2 I, and (CF.sub.3)SbI. Birich et al find that, with the exception of F.sub.3 PI and OPF.sub.2 I, the abovementioned column 5 compounds exhibit a two band absorption spectra, unlike the single band compounds C.sub.n F.sub.2n + 1 I I and CH.sub.3 I, although absorption in only one band may be effective in producing I*. For example, (CF.sub.3).sub.2 AsI has absorption peaks at approximately .lambda. = 2200 A and .lambda. = 2850 A, while CF.sub.3 I and C.sub.3 F.sub.7 I have one such peak each at .lambda. = 2650 A and .lambda. = 2760 A, respectively: The absorption peaks of the column 5 compounds appear to bracket the single band compounds such as C.sub.n F.sub.2n + 1 I. A further difference was noted: n-C.sub.3 F.sub.7 I, as representative of a generic class of iodine compounds CnF.sub.2n + 1 I, appears to degrade rather quickly through thermal decomposition, while compounds such as (CF.sub.3).sub.2 AsI can withstand perhaps 25 lamp flashes with only a 25% reduction in laser output. This suggests that the column 5 iodine-containing compounds may be superior for purposes of repetitive pulsing by a fluorescent source.
The abovementioned laser candidates have been driven photolytically, and the subject invention accomplishes this by e-beam driven fluorescence in noble gas-halide mixture, in pure noble gases, in pure halide gases, and other fluorescers responsive to e-beam excitation.
Velazco and Setser, in Journal of Chemical Physics, Vol. 62, p. 1990 (1975), have studied bound-free and bound-bound emission spectra of excited xenon halides, which subsequently radiate to the dissociative or very weakly bound ground state (xenon + halide) with the emission of ultraviolet light (.lambda. .congruent. 3,000 A). The compounds XeI*, XeBr*, XeCl*, and XeF* were studied and their emission spectra were determined. The 4-10 eV energy radiated in these transitions is appropriate for photolytically driving the various classes of laser gas of the subject invention.
Mangano and Jacob, of the Avco Everett Research Laboratory (28th Gaseous Electronics Conference Proceedings, Oct. 20 - 24, 1975, University of Missouri at Rolla) have studied electron beam discharge pumping of the KrF laser, using 98% Ar as a buffer in a 1 atm. mixture. Apparently rare gas metastable production fluorescence efficiency is about 50% with direct electron beam pumping against a possible 70% efficiency with discharge pumping. Finally, they find that if Kr* can be produced with 75% efficiency in a discharge, the possible laser efficiency of the e-beam driven KrF is 5-10%.
Hughes, Shannon and Hunter, in Applied Physics Letters, Vol. 24, p. 488 (May 1974) and Vol. 25, p. 86 (July 1974), have reported superfluorescence response from noble gases such as pure Xe and Ar as well as Xe-Ar and Xe-Ne mixtures, driven by an electron beam. Their calculations indicate a 1% conversion efficiency of the laser system, amounting to about 60 joules/liter radiation in the ultraviolet with their 800 keV e-beam input of 6 joules/cm.sup.3 and 300 amps/cm.sup.2. This superfluorescent radiation is also appropriate for photolytically driving a secondary laser cell.
The emission spectrum of XeI* in e-beam excited Xe/I.sub.2 mixtures has been reported by Ewing and Brau, Physical Review A, Vol. 12, p. 12 (July 1975). The mixture of Xe and I.sub.2 with an Ar buffer was irradiated with a 400 keV electron beam, which causes a recombination of Xe and I into bound excited states on a time scale of nanoseconds. The radiation observed is believed to arise from decay of XeI*(.sup.2 .SIGMA.), resulting in emission of a 2560 A photon. This radiation, again, is in the appropriate range for driving the laser gas, as discussed above.
Finally, Searles and Hart, In Applied Physics Letters, Vol. 27, p. 243 (August 1975), have reported laser emission at 2818 A from XeBr* directly driven by an e-beam, the laser radiation coming from decay of the XeBr* molecule. E-beam energy reported was in excess of 400 keV, and total pressure used was about 5000 torr. The Searles and Hart system, of course, was concerned with direct e-beam driving of the fluorescer gas of the subject invention, for purposes of laser action.
The foregoing results indicate that: (1) an e-beam may be used to drive a substance such as a rare gas halide or a pure noble gas or halide gas with the emission of ultraviolet radiation: (2) the e-beam pumping may achieve laser efficiencies of 5 - 10%, with the prospect of higher fluorescer efficiencies; and (3) ultraviolet radiation may be used to pump, for purposes of lasing action, classes of compounds including the column 6 elements, the cyanogens, and certain iodine-containing compounds, such as CF.sub.3 I and (CF.sub.3).sub.2 AsI.